Proton NMR methods are used to probe the binding of pepstatin and analogs of pepstatin to pig pepsin. Difference spectra reflect changes in enzyme conformation induced by the peptides and broadening of specific resonances due to spin labeled peptides. Preliminary experiments have shown the feasability of incorporating 3-fluorotyrosine into the acid protease of Rhizopus chinensis. F-19 NMR experiments are planned.